基于日期的多维数组排序

2-4-2020,bat
14-3-2020,ball
25-6-2019,purse
27-8-2019,rice

<?php
$newdates=$mydates=array();    
$lines = file('n:/mydata.txt');    
foreach($lines as $data)
{
    $mydates[]=explode(',',$data);
}  

$countdates=count($mydates);    
for ($i=0;$i<$countdates;$i++){
    $chkdate=$mydates[$i][0];
    $currentdate=DateTime::createFromFormat('d-m-Y', $chkdate)->format('d-m-Y');
    $mydates[$i][0]=$currentdate;
}  
array_multisort($mydates);
$newdates=$mydates;    
$file = fopen("n:/newdates.txt","w");
foreach ($newdates as $contentline)
{
    fputcsv($file,$contentline);
}    
fclose($file);